A cryogenic engine is an engine which uses a working fluid (WF) and a heat exchange fluid (HEF) at an elevated temperature relative to the WF to transfer heat to the working fluid. The cryogenic engine introduces the working fluid to an expander of the engine which expands the working fluid to do work.
In order to transfer heat to the working fluid, the HEF comes into thermal contact with the working fluid. The HEF is generally mixed with the working fluid and then recovered. The cryogenic engine may additionally comprise a heat exchanger for transferring heat to the working fluid. The working fluid and the HEF may be introduced to the expander separately, where they become mixed, and/or the HEF may come into thermal contact with the working fluid before the working fluid is introduced to the expander.
The working fluid may be stored at very low temperatures before heat is transferred to the working fluid. By “very low temperatures” is meant temperatures at which gases such as air, nitrogen, oxygen and natural gas are in a liquid phase at atmospheric pressure. Thus the storage temperature is always less than about −150 degrees Celsius. However, once heat has been transferred to the working fluid, the working fluid is at a temperature above the storage temperature, usually significantly above the storage temperature, and most usually at or near to ambient temperature, which is in a range of from about +5 to about +25 degrees Celsius, although it may be at a temperature below 0 degrees Celsius. For refrigeration-related applications, the working fluid is usually in a range from about 0 to about +30 degrees Celsius and for waste-heat recovery applications, in a range of from about +60 to about +100 degrees Celsius.
The working fluid may be a liquefied gas as it is introduced to the expander, and the expander may then vaporize the working fluid, or the working fluid may already be in the vapour phase but under pressure or in a supercritical state before it is introduced to the expander. By a “supercritical state” is meant that the working fluid may be at a temperature and pressure above its critical point in the fluid's phase diagram, where distinct liquid and gas phases do not exist. Thus the expansion may involve a phase change of the working fluid from liquid to vapour, or if the working fluid is already in the vapour phase and under pressure or in a supercritical state before being introduced to the expander, it need not involve a phase change.
Ideally, the heat transferred to the working fluid by the HEF is equal to the heat which would otherwise be lost by the working fluid during its expansion, so that the expansion of the working fluid is isothermal. This is in contrast to a steam engine and to an internal or external combustion engine, for example, all of which operate by ideally adiabatic expansion of a working fluid to do work.
The present invention is a development of the cryogenic engine system described in U.S. Pat. No. 6,983,598 (Dearman 001). This engine includes one or more cylinders and a piston in each cylinder and employs a source of working fluid (WF), normally comprising a gas derived from a liquid cryogenic source, which is introduced into a chamber of the engine in combination with a heat exchange fluid (HEF) which transfers heat to the working fluid (WF) such as to cause a higher degree of expansion of the working fluid (WF) within the chamber than would otherwise be possible. The expansion of the working fluid (WF) is used to drive the piston which in turn drives an output shaft such as to produce useful shaft horsepower. The engine includes inlet and outlet valves for each of a number of cylinders and these are controlled such as to ensure both working fluid and HEF are supplied to the cylinder before the inlet valves are closed. The description provides for a flow control device which may be a timed injection pump which is operative to dispense dosages of working fluid (WF) at appropriate points of the cycle of the engine. In the example given, during the first (expansion) part of the cycle, heat exchange fluid (HEF) is drawn into the cylinder through the inlet valve and at that point working fluid (WF) is also injected into the cylinder. The working fluid is exposed to the heating effect of the heat exchange fluid (HEF) and expands and the pressure in the cylinder rises such as to cause the piston to undertake an expansion stroke. When the piston reaches bottom dead centre (BDC), an exhaust valve is opened and the expanded working fluid (WF) and heat exchange fluid (HEF) is expelled from the cylinder and routed towards a separator and reservoir for future re-use. In this arrangement, the heat exchange fluid (HEF) is drawn into the cylinder during the first part of the expansion portion of the cycle, implying the heat exchange fluid (HEF) valve is opening at or around TDC and closing sometime post TDC.
It has been found that effective control of Heat Exchange Fluid (HEF) introduction into the expansion chamber is essential to the efficient operation of the engine concept. Engine testing shows that introduction of HEF during the first phase of the expansion stroke does not allow for efficient expansion. This is because the injection of working fluid must be shifted to later in the expansion stroke where the high rate of expander volume change reduces the volumetric efficiency due to valve flow limitations.
Therefore, there is a need for an improved engine system which overcomes these issues.